Conventional radiographic procedures, in which x-rays from a fixed origin point pass through a subject to a broad photographic film or florescent screen, have serious limitations and disadvantages. Only a small proportion of the x-rays which reach the film or screen cause an actinic or florescent reaction. Consequently, an undesirably high radiation dosage of the subject, which may be a medical or dental patient for example, is necessary to obtain the desired data. Where film is employed, the image is not immediately available as the developing process requires time and specialized equipment.
Further, both photographic film and florescent screens have a very limited contrast range or gray scale capability in comparison with the full range of x-ray transmissivity variations between different regions of many subjects. Consequently, the full range of information which is potentially available from a given exposure of a subject is not visible or detectable in the film or screen. An exposure of a medical patient may, for example, be arranged to produce a film or florescent screen radiograph depicting dense bone structure. Owing to the limited contrast range of the film or screen, it is not possible to perceive, in the same image, a region of soft tissue such as a tumor that has an x-ray transmissivity differing only slightly from the adjacent soft tissue. If the exposure is adjusted to depict contrast differences between regions of soft tissue, than details of the relatively radiopaque bone structures are not present in the image. Consequently, repeated exposures of the subject may be necessary to obtain all of the information which may be needed. This further increases the radiation dosage of the subject.
These problems can be greatly alleviated by utilizing scanning x-ray apparatus for producing radiographic image data, an example of such apparatus being described in U.S. Pat. No. 3,949,229 issued Apr. 6, 1976 to Richard D. Albert for X-RAY SCANNING METHOD AND APPARATUS. In a scanning system of this kind the subject is situated between an x-ray source which generates a moving x-ray origin point, in response to raster sweep signals, and at least one relatively small x-ray detector. The detector produces signals that vary in accordance with variations of x-ray transmissivity within the subject. An X-Y display device, such as a cathode ray tube oscilloscope or a television receiver, may be provided with sweep signals corresponding to those of the x-ray source while the Z or intensity signal for the display device is derived from the x-ray detector signals. A radiographic image of the scanned region of the subject is then produced at the screen of the display device.
The scanning x-ray apparatus can produce instantaneous images and greatly reduces radiation dosage of the subject since electronic x-ray detectors respond to a much larger proportion of intercepted x-rays than film or florescent screens.
Further, the scanning system enables extraction and display of a much greater range of information from a single exposure of the subject. Although the display screens of cathode ray tubes or the like also have a very limited contrast or gray scale range in relation to the full range of x-ray transmissivity variations encountered in the scanning of many subjects, the x-ray detector signals may be transmitted to a plurality of such display means with each being adjusted to image a different limited contrast range selected from the full range of contrast information present in the detector signals. U.S. Pat. No. 4,032,787 issued June 28, 1977 to Richard D. Albert and entitled METHOD AND APPARATUS PRODUCING PLURAL IMAGES OF DIFFERENT CONTRAST RANGE BY X-RAY SCANNING discloses one form of signal processing circuit with which detector signals from a single exposure of a subject may be used to simultaneously display a plurality of images each of which depicts a different selected range of x-ray transmissivity variations within the scanned region of the subject.
Scanning x-ray systems also enable processing of the detector signals to impart advantageous characteristics to the radiographic images. As described in the above identified U.S. Pat. No. 3,949,229, with particular reference to FIG. 13 thereof, a feedback signal circuit may be provided between the x-ray detector and x-ray source which varies the production of x-rays at the source in such a manner that the average level of the detector signals is maintained substantially constant while regions of the subject that may have greatly different x-ray transmissivities are scanned. This suppresses certain data from the detector signal in order to emphasize other desired data. More specifically, the detector signal then varies only in response to scanning across abrupt boundaries between regions of the subject having different x-ray transmissivities. Changes of transmissivity which occur relatively gradually in the course of the scan and sustained changes are not represented in the detector signal. The result is a highly advantageous edge enhancement effect which makes the outlines of regions of the subject of differing transmissivity readily apparent in the image. Regions which may have only slightly differing transmissivities are readily apparent as it is primarily the abruptness of a transmissivity change rather than the magnitude of the change that varies the detector signal.
The advantageous effects of the feedback circuit are gained at the cost of suppressing information, which would otherwise be available, from the output of the x-ray detector. The feedback circuit may eliminate, from the detector signal, a very large amount of contrast or transmissivity variation data pertaining to the scanned region of the subject. Thus, using known systems, it is not possible to produce specialized images such as the edge enhanced image discussed above while also producing from the same radiation exposure of the subject certain other highly useful images such as plural images of different contrast range selected from the total range of contrasts present in the scanned region of the subject.
Thus, using known systems for processing signals from scanning x-ray apparatus, the obtaining of certain kinds of desirable image information has the effect of interfering with obtaining of other kinds of useful image information from the same exposure of the subject. To minimize radiation dosage and to enable the extraction of a greater variety of useful image information from a given scanning of the subject, it is preferable that signal processing means be available for simultaneously generating plural image signals of different informational content including image signals indicative of data which has been suppressed from the output of the x-ray detector in the process of producing others of the image signals.
The scanning x-ray systems discussed above are of a type primarily used for producing shadowgraph images. Essentially similar problems can be encountered in certain other types of x-ray scanning system such as computed axial tomograph x-ray scanners of the type which produce an essentially cross-sectional image of the scanned subject.